This invention relates to a process for the preparation of acrylic core/shell polymers. It has been known that acrylic core/shell polymers, wherein a hard shell of poly (lower alkyl methacrylate) homo- or copolymer is polymerized in the presence of at least an equal weight of a rubbery homo- or copolymer of an alkyl acrylate, are useful as impact modifiers for many thermoplastics, especially poly(vinyl chloride). U.S. Pat. Nos. 3,655,825 and 3,678,133 are exemplary of such art. With suitable modification, such modifiers are useful to impart impact to such substrates as poly(methyl methacrylate), styrene-acrylonitrile copolymers, poly(caprolactam), etc. U.S. Pat. Nos. 3,668,274, 3,796,771, 3,793,402, 3,808,180 and 3,843,735 are exemplary of such art.
It has been further known that the ratio of rubbery to hard stage should be as high as possible to make the modifier as efficient as possible in improving the impact strength of the plastic matrix. However, when the acrylic rubber level exceeds 70 % by weight of the total polymer weight, it has previously been impossible to isolate the modifiers by spray-drying to yield a free-flowing powder at isolation rates which are practical.
Spray-drying, in which the polymer in aqueous dispersion is atomized in a chamber containing heated air, is an economical, safe and desirable means of isolating such acrylic core/shell polymers in particulate free-flowing form. Attempts to isolate by spray drying modifiers containing 70 % rubber phase were thwarted by the pronounced tendency of the modifier to stick to the walls, to bridge over conveying lines entrances, and to exhibit unacceptable powder flow behavior marked by aggregation, lumping, and flow interruptions. Even acrylic core/shell polymers with at least 55 % by weight rubber core were spray dryable only at limited throughput rates, and frequently buildup on chamber walls and floor blockages occurred. Attempts to improve powder flow have been varied. The ratio of rubber to hard phase has been reduced or a hard polymer in emulsion form has been co-spray dried. The flow behavior is improved, but at the expense of impact efficiency. Addition of inorganic additives to the emulsion has not been effective in improving flow except at levels high enough (5 % or above) to cause severe mixing and pumping problems. Post-addition of flow aids to the dried material is effective but undesirable, calling for an additional and expensive blending step. It is an object of this invention to provide a process of isolating acrylic core/shell polymers of high rubber content by spray drying.
It is a further object to produce acrylic core/shell polymers of at least 55 % by weight rubber content at increased spray dryer output levels. These objects and others as will become apparent from the following are achieved by the present invention which comprises spray-drying an acrylic core/shell polymer having at least 55 % by weight rubbery core while blowing inert particulate material into the gaseous drying medium.
The acrylic core/shell polymer has at least two stages produced sequentially in emulsion. The first or core stage rubbery latex is preferably poly (n-butyl acrylate) but may be based on ethyl, isobutyl, 2-ethylhexyl, or other lower alkyl acrylates or copolymers of the same. The rubbery core polymer optionally contains up to 20% of other monounsaturated or conjugated dienic copolymerized monomers, such as styrene, vinyl acetate, vinyl chloride, methyl methacrylate, butadiene, isoprene, and the like and optionally up to 5% of a crosslinking polyunsaturated monomer having two or more non-conjugated sites of approximately equal reactivity, such as ethylene glycol diacrylate, butylene glycol dimethacrylate, divinyl benzene, and the like. It also optionally contains up to 5% of a graft-linking monomer having two or more non-conjugated sites of unsaturation of unequal reactivity, such as diallyl maleate, monoallyl fumarate, allyl methacrylate, and the like.
The shell stage, which is optionally chemically grafted to the core rubbery stage, is preferably polymerized from methyl methacrylate and optionally other lower alkyl methacrylates such as ethyl or t-butyl or mixtures of lower alkyl methacrylates. Further, up to 40% by weight based on total shell stage monomers can be other monovinylidene monomers, such as styrene, vinyl acetate, and vinyl chloride, and the like. Agents to control the molecular weight, such as alkyl mercaptans, can be present in either or both stages. The acrylic core/shell polymer is prepared in emulsion, and is isolated from the resulting latex by spray drying in accordance with the invention.
The spray-drying can be conducted in any of a variety of designs. The latex is atomized either by a wheel or by a nozzle, and the drying gas can enter from the top or the bottom of the atomization chamber. The drying gas is preferably heated air or nitrogen. Inert particulate material is blown into the atomization chamber by a separate gas stream or fed into the heated drying gas stream at such a rate to give the desired weight on dry acrylic core/shell polymer solids.
It is preferred that the inert particulate material be fumed silica, such as Cab-O-Sil M-5 (Cabot Corporation) of particle size 0.012 microns. Other inert materials of similar particle size may be used, such as calcium carbonate, titanium dioxide, poly(methyl methacrylate). The additive should be chemically inert, non-reactive with either the core/shell polymer, the thermoplastic matrix, or any other additives, such as stabilizers, lubricants, etc., for that matrix.
Levels of inert particulate material are preferably about 0.01-1.5%, more preferably about 0.1-1.0%, and most preferably about 0.25-0.75%, based on dry weight of acrylic core/shell polymer. Higher levels will achieve the desired improved drying and flow characteristics, but with deleterious effects on physical or optical properties of the modifier/matrix blend, such as clarity, impact strength, melt flow properties, etc.
It has been known to use such additives to improve the flow properties of powder blends, but by dry-blending polymer and particulate. It is also known to add particulate materials in a spray dryer during isolation of natural rubber latex, but at much higher levels and with a necessary adjustment of pH (U.K. Pat. Nos. 723,315 and 901,167); none teach or suggest the addition of such low levels of inert particulate material and their dramatic effect on powder flow without detriment to the other properties of the polymer. The present invention does not involve a pH adjustment step as in the U.K. patents.
The core/shell polymers produced by this process are useful for modifying PVC as well as other thermoplastic polymers such as nylon, polymethyl methacrylate, and others.